The electrophotographic method for forming a visible image comprises forming an electrostatic latent image on an electrostatic image-carrying substrate comprising a photoconductive light-sensitive material by charging and exposing thereof to light; developing the latent image with a toner composition comprising a binder resin and a coloring agent and transferring and fixing the toner image obtained to a support, e.g., a transfer paper.
As the method for developing electrostatic images, a method in which to an electrostatic latent image formed on a light-sensitive material are attached developer particles (toner) having triboelectric charge of the opposite polarity to the latent image through Coulombic attraction (normal development), and a method in which toners having triboelectric charge of the same polarity as of the latent image are attached by an electric field between a magnetic developing brush and a light-sensitive material (reversal development) are known. The light-sensitive material is charged positively or negatively depending on the type thereof and is exposed to light corresponding to information to form an electrostatic latent image and, thereafter, the latent image is developed by the normal or reversal developing method. Thus, the polarity of the toner to be used is determined by a combination of the charged polarity of the light-sensitive material and the normal or reversal development. For example, in a laser beam printer in which imagewise exposure is applied using laser beam to selenium (negatively charged) to carry out the reversal development, a positively chargeable toner is used.
A toner image formed on the light-sensitive material is transferred to paper, overhead projector (hereinafter referred to as "OHP") sheets, and so forth and then fixed by utilizing heat, pressure, solvent vapor or electromagnetic waves. As the fixing method, a heat roller system using heat and pressure in combination is recently most widely used because it is excellent in heat efficiency.
As the toner composition to obtain a visible image, a composition comprising a binder resin and a black coloring agent such as carbon black dispersed therein is commonly used. In recent years, color toners prepared by dispersing cyan pigment, magenta pigment or yellow pigment in a binder have been used. Among the color toners which are used to obtain a panchromatic image by the electrophotographic method, usually color toners of cyan, magenta and yellow colors are used.
For such color toners are required to project a color image formed by transferring and fixing on an OHP sheet on a screen by a transmitted light through OHP.
The color toner generally comprises a binder resin, a coloring agent as main components and various additives. As the binder resin, polystyrene, a styrene-(meth)acrylic acid ester copolymer, a styrene-butadiene copolymer, polyester, an epoxy resin, a cumaron-indene resin and the like are generally used. In particular, a polyester resin is firstly used due to relatively good in fixing properties and good in transmitting properties through OHP.
The polyester resin, however, is inherently negatively chargeable and, therefore, for positive charging it is necessary to add a charge-controlling agent. As the charge-controlling agent for positive charge which is added, nigrosine dyes are widely used. Since, however, the nigrosine dyes produce strong coloration and cannot produce an inherent color of the coloring agent they therefore, cannot be used in the color toner. As a colorless or light colored charge-controlling agent, various quaternary ammonium compounds have been proposed. These quaternary ammonium compounds are not sufficiently effective in permitting positive charging, and strongly negatively chargeable toners are produced from the polyester resin, thereby suffering from problems such as scattering of toners from a developing machine and producing fog on the image.
To achieve suitable positive charging, a method of adding a separately synthesized amino group-containing vinyl polymer to the polyester resin and kneading them together is described in JP-A-61-105562 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). However, compatibility of the polyester resin with an amino group-containing vinyl polymer is not necessarily high. Thus, micro phase separation occurs at the time of kneading and cooling in production of toners, and negatively chargeable toners are formed owing to the inherent nature of the polyester resin, thereby causing problems such as fogging of a base, scattering of the toner, and unsatisfactory transfer. Furthermore, the micro phase separation deteriorates the light transmitting properties of the toner, and a problem arises in that when an OHP sheet is used, a transmitted image which is blackish and of low saturation is obtained.